CONCRETE CRACKING –WHO IS TO BLAME?

ByChristopher Stanley

TECHNICAL DIRECTORUNIBETON READY MIX

CRACKING FACTS(Concrete Society Technical Report Number 22, non-structural cracks in concrete)

“If a concrete is either cooled or dried, then provided it is free from restraint, it will reduce in length and no cracks will

develop”

“Cracks will not form unless there is some form of restraint”

“Because it acts as a form of internal restraint, reinforcement governs the spacing and width of cracks in hardened concrete

but reinforcement does not have the same effect in plastic concrete”

CRACK ASSESSMENTUsually based on:

Critical viewing distance and personal viewpoint

Type of structure, often using an arbitrary or “prestige”scale (e.g monumental or public buildings, commercial buildings and car parks, public paving, driveways, private housing)

Cracks can be classified thus

Fine cracks - up to 1mm wideWide cracks - from 1mm to 6mm wideFractures - over 6mm wide

It is sometimes specified or implied that crack widths of up to 0.3mm are aesthetically acceptable

All concrete cracks but some can be prevented

•

CRACKS OCCURINGBEFORE HARDENING

CONSTRUCTION MOVEMENT

SUB-GRADE MOVEMENT

FORMWORK MOVEMENT

CRACKS OCCURING AFTER HARDENING

STRUCTURAL CRACKS

DESIGN LOADS

ACCIDENTAL OVERLOAD

CREEP

EXTERNAL SEASONAL TEMPERATURE VARIATIONS

FREEZE/THAW CYCLES

CHEMICAL

CEMENT CARBONATION

ALKALI-AGGREGATE REACTIONS

CORROSION OF REINFORCEMENT

SHRINKABLE AGGREGATE

CRACK CLASSIFICATION

PLASTIC CRACKS

PLASTIC SHRINKAGEPLASTIC SETTLEMENT

CRAZING

DRYING SHRINKAGE

PHYSICALCRACKS

EXTERNAL RESTRAINT

INTERNAL TEMPERATURE

GRADIENTS

EARLY AGE THERMAL

CONTRACTION

THERMAL CRACKS

TYPE OF CRACK -time of appearance REF * FORM, LOCATION, etc. PRIMARY/SECONDARY

CAUSES REMEDY * *

PLASTIC SETTLEMENT TYPE A Cracks over reinforcement

in deep sections Excess bleeding (PC) Reduce bleeding

10 minutes to three hours

TYPE B “Arching” cracks in columns Re-vibrate

TYPE C Cracks at change of depth in slab/beam sections

Rapid early drying conditions

Add Air entrainment

PLASTIC SHRINKAGE TYPE D Diagonal cracks in roads

and slabs Rapid early drying (PC)

30 minutes to six hours

TYPE E Random cracks in reinforced slabs Low rate of bleeding Improve early

curing

TYPE F Cracks over reinforcement in slabs

Ditto and steel near surface

EARLY THERMAL CONTRACTION TYPE G External restraint cracks

in thick walls or columnsExcess heat generation

(PC) rapid coolingReduce heat

and/or insulateOne day to three

weeks TYPE H Internal restraint cracks in thick slabs

Excess temperature gradients, rapid cooling

LONG TERM DRYING SHRINKAGE

weeks - monthsTYPE I

Cracking in thin slabsand walls

Inefficient joints (PC) Excess shrinkage and

inefficient curing

Reduce water content

Improve curing

CRAZINGTYPE J Cracks “off the form” in

fair-faced concreteImpermeable formwork, rich mixes, poor curing

Improve curing and finishing

1 - 7 days sometimes much later

TYPE K Cracks in power- floated slabs

over-trowelling.

SIMPLE CRACK MODEL

Initial state after pouring

r est r ai nt

If dried out or cooled with partial

or no restraint

rest

rain

tRestraint - shortterm effects

RestraintMedium/long term effects

Crack relieves tension

Contraction without stress

n o re

stra

int re s tra in t

Tension!

Free to shrink

Common crack types – Plastic shrinkage cracking (Type A)

Caused by BLEEDING (“A special case of sedimentation” - T.C.Powers, 1939)

The phenomenon of water rising to the surface of plasticconcrete, caused by gravity pulling heavier particlesdownward, the latter being known as sedimentation

Bleeding is not a result of poor compaction, and it cannot be eliminated by improved compaction

Sedimentation

bleed water evaporates - volume change - shrinkage - tension - restraint - crack?

PLASTIC SETTLEMENT CRACKSType A cracks

Water void formed under steel – desiccation - water

evaporates leaving dry void

Settlement cracks occurring at changes in slab depth

Type C cracks

PLASTIC SETTLEMENT CRACKS

Tension

Item cast into slab acts as crack

inducement - crack follows line of least

resistance

Uneven sub base acts as crack

inducement - crack follows line of least

resistance

PLASTIC SHRINKAGE CRACKS

Type E - random

Type F – over reinforcement

PLASTIC SHRINKAGE CRACKS

Insufficient reinforcement cover ?(cracks induced by steel proximity

to surface which is in tension due to rapid drying)

Differentiated from plastic settlementcracks because plastic shrinkage cracks

tend to pass through slab depth

Tension

Plastic Shrinkage Cracking

Plastic Shrinkage Cracking

Plastic Shrinkage Cracking

Plastic Shrinkage Cracking

Plastic Shrinkage Cracking

Thermal cracking - specificationQuestion - Why 70°C max. temperature?Past experience with mass concrete and accelerated curing e.g. in precasting suggests that the quality of the cement hydrate at elevated temperatures >70°C is inferior to that in a normally cured concrete so mechanical strength tends to be lower - in addition a phenomenon known as Delayed Ettringite Formation (DEF) may affect durability

Question - why require a maximum temperature differential of 20°C?

Raw materials for concrete expand at different rates when heated up - this may lead to “micro-cracking” when the respective coefficients of expansion of cement paste and aggregates are significantly different in some cases

Most aggregates can absorb a degree of strain from temperature movements (“tensile strain capacity”) therefore it does not automatically follow that aggregates with significantly different E. coef.are going to cause or influence cracking.

20°C taken as a conservative limit on differentials due to lack of knowledge of local aggregate expansion coefficients

20°C

EARLY THERMAL CONTRACTION CRACKS

Core temperature

Surfacetemperature

Higher Temperature differential -Possibility of

cracks?

Time after casting - hours

Tem

pera

ture

°C

>20°C?≤20°C?

Temperature differential

EARLY THERMAL CHANGES - 24 HOURS

Heat and Expansion

75°C?

Sections > 0.5m thick considered “Self-Insulating”

Insu

latio

n pr

ovid

ed b

y fo

rmw

ork?

Ambient temperature say 35°C?

Formwork temperature say 55°CCover to Insulate?

Insu

latio

n pr

ovid

ed b

y fo

rmw

ork?

Maybe base restraint from

mature concrete or sub base material

No edge restraint

Free to shrink?

EARLY REMOVAL OF FORMWORK - THERMAL SHOCK

Core Heat say 75°C

BASE RESTRAINT

Remove forms early?

Remove forms early?Expansion

Rap

id c

oolin

g to

am

bien

t te

mpe

ratu

re –

say

35°C

?

Rap

id c

oolin

g to

am

bien

t te

mpe

ratu

re –

say

35°C

?

Tension - Plastic shrinkage

Tension!Tens

ion!

METHOD CONTRIBUTION DISADVANTAGEPour concrete continuously

Improves uniformity of pouring temperatures

Places additional demands on production and

handling

Pour concrete at nightNegligible.Depends on

speed and volume placed and nature of hydration of

cement

Extra logistical considerations for night

working

Delay removal of formworkSignificant contributionPrevents thermal shock

Allows uniform controlled cooling to take place

Formwork re-use delayed

Use insulated curing methods

Significant reduction in temperature differentials.

Enables more uniform temperature rise and fall

Large scale use of insulation materials may be

expensive

METHODS EMPLOYED TO REDUCE EARLY AGE THERMAL CRACKING

METHOD CONTRIBUTION DISADVANTAGEInstall sacrificial cooling system in the concrete in

the concrete massDepends on efficiency of

cooling system

Very wasteful.Durability problems from

embedded pipes

Reduce design strength margin or observe 60 or 90

day compliance

Significance based on amount of cement reduced

28 day requirements?Durability may be

compromised

Use superplastisizing admixtures

Can significantly reduce cement content.

Significance dependant on amount of reduction

achieved

Extra cost of superplasticizer over

normal admixture cost

Use admixtures formulated for hydration control and/or significantly

increase dosage

Can significantly reduce cement content ,delay

hydration and reduce peak temperatures

Extra cost of admixture over normal admixtureSetting times extended

Formwork removal may be delayed

METHODS EMPLOYED TO REDUCE EARLY AGE THERMAL CRACKING

METHODS EMPLOYED TO REDUCE EARLY AGE THERMAL CRACKING

METHOD CONTRIBUTION DISADVANTAGE

Use chilled water to partly or wholly replace mixing

water

Significant- 4oC water temperature =

Approx - 1oC concrete temperature

Plant equipment expensive. Consumption usually

exceeds supply capacity. Storage tanks usually

required

Use ice to partly or wholly replace mixing water

Significant50% ice = approximately

- 10oC concrete temperature100% ice =

approximately - 17oC

Usually expensive. Handling difficult, must be

weighed.May not be available in

remote areas.

Nitrogen gas injection Depends on quantity of Nitrogen gas injected

ExpensiveHandling difficult in remote areas. Difficult to control.Efficiency lost due to gas

leakage to atmosphere during injection

METHODS EMPLOYED TO REDUCE EARLY AGE THERMAL CRACKING

METHOD CONTRIBUTION DISADVANTAGE

Shade stockpiles Significant

Exposed stockpile temperatures can reach >50°C in mid day direct

sunlight

Extensive shading required - combination of shading

and fresh deliveries of aggregates to manage

temperatures effectively

Water spraying of stockpiles

Depends on requirements of pour and aggregate

demand

Some difficulty in maintaining uniform, effective, large scale

spraying for mass pours -moisture uniformity can be

compromised

Control temperature of fresh cement

Influence of cement temperature not significant as volume of cement is only

about 12% of concrete mass

Logistical difficulties as large pour cement demand maximizes available silage

STOCKPILE TEMPERATURE MEASUREMENT

Base restraint from mature

concrete

No edge restraintFree to shrink?

Less “Self-insulation”from lower section

thickness

Sections > 0.5m thick considered “Self-Insulating”

Thermal contraction of concrete after

hardening

EARLY THERMAL CONTRACTION CRACKS

Internal restraintType H cracks

Type I cracks

LONG TERM DRYING SHRINKAGE CRACKS

Primary causes - Impermeable form-face materials

- Over-trowelling

Secondary causes

- Rich, pasty mixes

- Poor curing

- Thermal shock (application of cool water on hot surfaces)

Time of appearance - 1 - 7 days, sometimes much later

Remedial measures

Improve curing

Avoid over-trowelling

CRAZING

Type J cracks (crazing)

CRAZING

Heat of hydration

Heat of hydration

SIMPLE INSULATION OF FORMWORK for CONCRETE BREAKWATERS – GUAM 1998

Plastic cling film wrap

creates cells

Plastic cling film

wrap

Cell heats up to higher

temperature

Site LocationDate concrete placedGrade/type of concrete/slumpWeather/site conditions at time of pouring e.g dry, wet, sunny,changeable,cloudy, sunlight,

shaded, exposed

Temperature range ºC, (check met.reports?)Wind conditions/speed (check met.reports?)Relative humidity (check met.reports?)Curing system usedType of structureApproximate dimensionsImmediate sub-baseDetails of reinforcement especially top steel

When was cracking first noted – hours/days

Cement type Coarse aggregate Fine aggregate Admixture

kg/m³ 20mm kg/m³ (1) kg/m³ (1) mls/100kgFly Ash kg/m³ 10mm kg/m³ (2) kg/m³ (2) mls/100kg

CRACK FIELD REPORT